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Earth-Science Reviews
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Quaternary time scales for the Pontocaspian domain: Interbasinal
connectivity and faunal evolution
W. Krijgsman
a,⁎, A. Tesakov
b, T. Yanina
c, S. Lazarev
a, G. Danukalova
d, C.G.C. Van Baak
e,
J. Agustí
f, M.C. Alçiçek
g, E. Aliyeva
h, D. Bista
i, A. Bruch
j, Y. Büyükmeriç
k, M. Bukhsianidze
l,
R. Flecker
i, P. Frolov
b, T.M. Hoyle
a, E.L. Jorissen
a, U. Kirscher
m, S.A. Koriche
n,
S.B. Kroonenberg
o, D. Lordkipanidze
l, O. Oms
p, L. Rausch
q, J. Singarayer
n, M. Stoica
q,
S. van de Velde
r, V.V. Titov
s, F.P. Wesselingh
raDepartment of Earth Sciences, Utrecht University, Budapestlaan 17, Utrecht 3584, The Netherlands bGeological Institute of the Russian Academy of Sciences, Pyzhevsky 7, Moscow 119017, Russia cLomonosov Moscow State University, Moscow 119991, Russia
dRussian Academy of Sciences, Institute of Geology of the Ufimian Scientific Centre, K. Marx St. 16/2, Ufa 450077, Russia eCASP, West Building, Madingley Rise, Madingley Road, Cambridge CB3 0UD, UK
fICREA. Institut Català de Paleoecologia Humana i Evolució Social (IPHES), Universitat Rovira i Virgili, Tarragona, Spain gDepartment of Geology, Pamukkale University, Denizli 20070, Turkey
hGeological Institute of Azerbaijan (GIA), H. Javid Av. 29A, AZ1143, Baku, Azerbaijan
iBRIDGE, School of Geographical Sciences and Cabot Institute, University of Bristol, University Road, Bristol BS8 1SS, UK jSenckenberg Forschungsinstitut Senckenberganlage, Frankfurt 25 60325, Germany
kDepartment of Geological Engineering, Bülent Ecevit University, Incivez/Zonguldak 67100, Turkey lThe National Museum of Georgia, 3 Purtseladze St., 0107, Tbilisi, Georgia
mEarth Dynamics Research Group, Department of Applied Geology, WASM, Curtin University, Perth, Australia nDepartment of Meteorology and Centre for Past Climate Change, University of Reading, UK
oDepartment of Applied Earth Sciences, Delft University of Technology, Delft 2600, The Netherlands pFac. de Ciències Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain qDepartment of Paleontology, Bucharest University, Bălcescu Bd. 1, Bucharest 010041, Romania rNaturalis Biodiversity Center, P.O. Box 9517, Leiden 2300, The Netherlands
sRussian Academy of Sciences, Institute of Arid Zones, Chekhova 41, Rostov-on-Don 344006, Russia
A B S T R A C T
The Pontocaspian (Black Sea - Caspian Sea) region has a very dynamic history of basin development and biotic evolution. The region is the remnant of a once vast Paratethys Sea. It contains some of the best Eurasian geological records of tectonic, climatic and paleoenvironmental change. The Pliocene-Quaternary co-evolution of the Black Sea-Caspian Sea is dominated by major changes in water (lake and sea) levels resulting in a pulsating system of connected and isolated basins. Understanding the history of the region, including the drivers of lake level and faunal evolution, is hampered by indistinct stratigraphic nomenclature and con-tradicting time constraints for regional sedimentary successions. In this paper we review and update the late Pliocene to Quaternary stratigraphic framework of the Pontocaspian domain, focusing on the Black Sea Basin, Caspian Basin, Marmara Sea and the terrestrial environments surrounding these large, mostly endorheic lake-sea systems.
1. Introduction
The Black Sea and Caspian Sea basins are the present-day remnants of the ancient Paratethys Sea (Laskarev, 1924), an epicontinental water-mass that developed since the earliest Oligocene in central Eur-asia as the northern branch of the Tethys Ocean. It was separated from the southern, Mediterranean branch by the Alpine-Caucasus-Himalayan
orogenic belt that progressively formed by ongoing tectonic collision of the Eurasian plate with the African-Arabian and Indian plates (Rögl, 1999;Popov et al., 2006). In Oligo-Miocene times, the Paratethys Sea covered large parts of Europe and Asia, stretching from southern Ger-many in the west to western China in the east. A complex combination of Mio-Pliocene tectonic uplift, glacio-eustatic sea level fluctuations, and sedimentation by major deltaic systems, progressively filled the
https://doi.org/10.1016/j.earscirev.2018.10.013
Received 13 July 2018; Received in revised form 9 October 2018; Accepted 16 October 2018 ⁎Corresponding author.
E-mail address:w.krijgsman@uu.nl(W. Krijgsman).
Available online 18 October 2018
0012-8252/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
marginal sedimentary basins in the west and east. Consequently, the Paratethys sea drastically retreated, which influenced the regional cli-mate of Eurasia (Ramstein et al., 1997) and facilitated mammal (in-cluding hominid) migration between Africa, Asia and Europe (Bar-Yosef and Belmaker, 2011).
Throughout its entire history, Paratethys formed a series of re-stricted basins, separated by shallow, tectonically active, gateways, that were extremely sensitive to small climatic and tectonic variations (e.g., Popov et al., 2006;Palcu et al., 2017). The semi-enclosed basin con-figuration resulted in extreme palaeoenvironmental dynamics including anoxic, hypersaline, brackish to fresh water conditions. Some salinity regimes in the basin, that were supposed to require connectivity to the open ocean, have been a major scientific puzzle for many centuries (Fig. 1:Kircher, 1678). The long-lived isolated position of the basins, combined with the exceptional palaeoenvironmental conditions, cre-ated faunal communities that are endemic to the Paratethys region, and that waxed and waned through geological history (e.g., Harzhauser et al., 2002). They obtained maximum extension during the latest Messinian (Lago-mare) times ~5.5 Ma, when Paratethyan faunas oc-cupied the entire Mediterranean Basin as well (e.g.,Guerra-Merchán et al., 2010;Stoica et al., 2016). Today the Paratethyan faunas are at their minimum: the so-called Pontocaspian communities are now al-most entirely restricted to small enclaves in the major deltaic and es-tuarine systems of the northern Black Sea as well as the Caspian Sea (Grigorovich et al., 2003;Yanina, 2012a). The origin, evolution and migration of these characteristic Pontocaspian faunal elements is still not fully understood (e.g.,Wesselingh et al., 2008).
The Pliocene-Quaternary co-evolution of the Black Sea-Caspian Sea is dominated by major changes in sea/lake levels. These may have been driven by external components, resulting from opening and closing gateways to the Mediterranean or Arctic ocean, as well as by internal components where hydrological and climatic changes induced by gla-cial-interglacial cycles may have created periods of intermittent inter-basinal connectivity (e.g.,Badertscher et al., 2011;Yanina, 2014). A simplified scenario is that the two basins were isolated during low-stands, when individual water levels, environmental conditions, and faunal composition were largely determined by the local hydrological budgets. The two basins became connected during highstands of the Caspian Sea. During such periods, overflow of the Caspian Sea through the Manych low-land connection north of the Greater Caucasus enabled faunal exchange (Fig. 2). Environmental conditions became similar in both basins by mixing of the water masses and consequently migration and blending of the Pontocaspian fauna took place. Additionally, the Black Sea became connected to marine waters of the Mediterranean during interglacial highstands and the Pontocaspian biota were mar-ginalized. These marine transgressions did not reach the Caspian Basin. Sea/lake level changes in the Pontocaspian region have been gigantic. Over 1000 m lake level rise has been proposed for the late Pliocene Productive Series-Akchagylian transition in the South Caspian Basin (see van Baak et al., 2017 and references therein). Also on short time scales, lake level changes were very significant as shown by Caspian Holocene variations of ~100 m affecting historic settlements on the Caspian coast (Kroonenberg et al., 2007). The Quaternary sea/lake level history of the Black Sea – Caspian Sea domain is still enigmatic
Fig. 1. Ancient map of the Pontocaspian region afterKircher (1678), who in his “Mundus Subterraneus” already envisaged that the Caspian Basin must have been
connected to the open ocean to explain its relatively high salinity (> 10 ‰) today. The Caspian Sea is in fact an isolated long-lived lake since at least 2.6 Ma. Kircher considered a subterraneous channel to the Persian Gulf for the marine connection. The location of this marine connection is still enigmatic today.
and awaits consistent interpretations, although significant progress has been made recently by geochemical proxies (e.g. strontium ratios, oxygen isotopes) that may clarify the timing and amount of con-nectivity (Major et al., 2006;Badertscher et al., 2011).
Understanding the fundamental mechanisms and processes that influenced both the geological and historical changes in sea level, connectivity, climate, and environment (e.g. salinity, anoxia, etc.) is also crucial for a coherent understanding of the future economical and sustainable developments in the region. The immensely rich hydro-carbon fields of both the Black Sea and especially the South Caspian Basin are the product of changing interbasinal connectivity, that gen-erated the anoxic source rocks of the Oligocene Maikop Series, the deltaic reservoir rocks of the Pliocene “Productive Series” and the brackish water cap rock of the Plio-Pleistocene Akchagylian clays (e.g., Hinds et al., 2004;Vincent et al., 2010). Paleoenvironmental changes in the region are causing the biodiversity crisis that the Pontocaspian fauna is experiencing today (e.g., (Grigorovich et al., 2003;Popa et al., 2009).
One of the key problems to understand the complex and intertwined geological history of the Pontocaspian region is the absence of reliable stratigraphic correlations between the Black Sea and Caspian Sea and between the lake/marine and continental domains. The lack of open marine faunal assemblages in the Paratethys generally hampers a straightforward correlation to the standard geological time scale, and the presence of mainly endemic faunas resulted in regional time scales for the different Paratethyan subbasins (e.g., Hilgen et al., 2012; Nevesskaya et al., 2003). For the Quaternary, individual time scales have been developed for the Caspian and the Black Sea region, both mainly based on their own characteristic faunal (usually mollusc) as-semblages from rich, but local sites (e.g., Yanina, 2014).
Cross-correlation has mainly been done on biostratigraphic arguments, be-cause radiometric, magnetostratigraphic and astronomical data are scarce and/or their results controversial. Carbon dating has been thoroughly applied to Holocene rocks (e.g., Yanina, 2014and refer-ences therein), but radiometric datings (K/Ar or Ar/Ar) of older Qua-ternary rocks are very rare, because of the common lack of intercalated volcanic ashes (e.g., Chumakov and Byzova, 1992). Magnetostrati-graphic correlations to the geomagnetic polarity time scale have been widely produced for the lower Quaternary successions (e.g., Molostovsky, 1997). They are more problematic for rocks younger than 780 kyr (last full reversal of the magnetic field), because there this technique only works if it is possible to sample with a resolution that is high enough (< 1 ka) to pick up the reversal excursions of the Brunhes chron (Laj and Channell, 2007; Singer et al., 2014). Consequently, many of the regional stage boundaries are still poorly dated and serious age uncertainties exist between the current geological time scales for the region.
Here, we present a comprehensive overview of the existing strati-graphic and geochronologic data for the Caspian Sea, the Black Sea and adjacent continental domains. The main result will be an update of the Quaternary geological time scale for the Pontocaspian region which will allow better understanding of the succession of geological events, especially dealing with the major changes in interbasinal connectivity and faunal evolution. We will furthermore discuss the state-of-the-art on the existing techniques and mechanisms that allow a better under-standing of Quaternary paleoenvironments, interbasinal connectivity, fauna migration patterns and sea level change in these long-lived anomalohaline lake systems, and the corresponding environmental changes in the terrestrial counterparts.
Fig. 2. Present-day drainage area of the Pontocaspian domain. Yellow circles denote the locations of the stratotype sections of the main Quaternary stages of the Caspian Basin and Black Sea Basin: 1) Akchaghylian on Krasnovodsk peninsula (Turkmenistan), 2) Apsheronian on Apsheron Peninsula (Azerbaijan), 3) Bakunian in Baku (Azerbaijan), 4) Kuyalnikian (Ukraine), 5) Gurian (Georgia), 6) Chaudian on Cape Chauda (Crimea) and 7) Uzunlarian (Crimea). White circles denote the locations of key sections: 1) Pyrnuar (N38.93, E56.26), 2) Malyi Balkhan (N39.27, E54.97), 3) Yuzhny Urundzhik (N39.27, E54.50), 4) Ushak (N40.45, E53.37), 5) Lokbatan (N40.33, E49.75) and Jeirankechmez (N40.24, E47.09), 6) Duzdag (N40.70, E46.92) and Bozdag (N40.80, E46.84), 7) Pantashara (N41.23, E46.36) and 8) Kvabebi (N41.48, E45.68) and Kushkuna (N41.25, E45.44).
2. Quaternary time scales of the Pontocaspian domain 2.1. The Caspian Sea region
The Caspian Sea is a lake: it is the world’s largest endorheic water body (Fig. 2) extending over 1200 km in latitude (36°-47°N), and 195-435 km in longitude (46°-56°E). The surface area and water volume of the Caspian Basin critically depend on the regional hydrological bal-ance. The Caspian Sea is divided into three subbasins of roughly similar surface area, but widely differing in depth and volume. The North Caspian Basin (< 15m deep, 1% volume) is separated by the Man-gyshlak sill from the Middle Caspian Basin (< 800m, ~33%) which is in turn separated by the Apsheron threshold from the South Caspian Basin (< 1025m, ~66%) (Panin et al., 2005;Zonn et al., 2010). At present, Caspian water level is ~27 m below global sea level, which gives a surface area of ~371,000 km2and volume of ~78,200 km3. The Caspian Basin is a huge reservoir of anomalohaline (often referred to as brackish) water. It is highly sensitive to climatic changes in its catch-ment area (3.5 million km2), which extends far northward to the central part of the East European Plain (Panin et al., 2005;Zonn et al., 2010). During the Quaternary, the catchment extended to include almost en-tire Central Asia. Today, the Caspian catchment contains forests and steppes in the Volga and Ural valleys and mountainous forests and arid regions in the Caucasus and Transcaspian areas. The salinity of the present-day Caspian Sea changes from 1‰ near the Volga delta in the north to 13.5 ‰ in the south (Dumont, 1998). The Volga discharge provides 85-90% of the total fresh water influx and forms the main element in the hydrological budget (Agapova and Kulakova, 1973; Mamedov, 1997;Zonn et al., 2010).
In late Miocene (Pontian) times the Caspian Basin was still con-nected to the Black Sea, forming the final phase of the ancient Paratethys Sea (Popov et al., 2004, 2006;Krijgsman et al., 2010). The Caspian Basin became isolated from the Black Sea in the earliest Plio-cene (Van Baak et al., 2016a), when a major drop in its water level resulted in far southward retreat of lake environments and an asso-ciated progradation of the Volga’s fluvio-deltaic deposits that reached the South Caspian Basin (Fig. 3a). This so-called Productive Series is the main South Caspian hydrocarbon reservoir unit (e.g., Hinds et al., 2004). The nature and extent of lake conditions in the southern basin at the time are unknown as these deposits are often several km below surface. Since its isolation from the Black Sea and open ocean, the Caspian Basin has experienced numerous transgressions and regressions with water level fluctuations of several tens to hundreds of meters re-sulting in enormous changes of its shoreline, especially in the flat northern part (Varuschenko et al., 1987;Svitoch, 2010a;Yanina, 2014). Our review of the Caspian stratigraphy starts with the late Pliocene Akchagylian transgression that resulted in the first of several lake phases that extend all the way to the modern Caspian Sea. Here we review the definitions and commonly used Plio-Quaternary strati-graphic subdivisions in the Pontocaspian domain, including their geo-chronological constraints, which allow a detailed correlation of the Caspian Sea with the Black Sea and the terrestrial and global ocean records.
2.1.1. Akchagylian
2.1.1.1. Description. During the Akchagylian age (late Pliocene-earliest Pleistocene) the largest Caspian transgression occurred, with shores extending well into the middle Volga and southern Urals to the north as well as the Sea of Azov in the west and the Aral Sea in the east (Fig. 3b). The Caspian Basin was a saline lake with major endemic faunal radiations but also events occurred that saw the introduction of marine foraminifera. The widespread fine grained intervals of the Akchagylian form the hydrocarbon cap rocks in many places in the Caspian Basin.
The Akchagylian stage was defined byAndrusov (1912), who de-scribed these deposits from the Krasnovodsk Peninsula on the eastern
side of the Caspian Sea (Andrusov, 1889, 1902, 1906) and named this stage after the Akchagylian district (Fig. 2). Andrusov did not designate a stratotype, and later the Ushak well was proposed as lectostratotype (a stratotype used to describe a stratigraphic unit that does not have a sufficient holostratotype). The stratigraphic profile of the Ushak well was thoroughly studied by many authors; the Akchagylian deposits are around 100 m thick but the base is not exposed (Dvali et al., 1932; Ali-Zade, 1961;Cheltsov, 1965;Nevesskaya, 1975a;Danukalova, 1996). In the stratotype area, Akchagylian deposits transgressively overlie Neo-gene red-colored rocks and are in turn overlain by Apsheronian lime-stones (Ali-Zade, 1961). In the Ushak profile, Ali-Zade (1961) and Cheltsov (1965)determined: 1) a 21 m lower unit consisting of alter-nations of calcareous clays, shelly limestones, calcareous sandstones and two thin layers of volcanic ashes, 2) a 25 m middle unit comprising grey sandstones with ferruginous concretions, and 3) a 59 m upper unit consisting of an alternation of clays, siltstones, sandstones and shelly limestones. Because the Akchagylian in the Ushak well section has no clear boundaries with underlying and overlying units, Danukalova (1996)suggested the Malyi Balkhan section in western Turkmenistan as neostratotype (astratotypedesignated after theholo/lectostratotypeas a replacement for it) and the Pyrnuar section as hypostratotype (an additionalstratotype, in different geographic context). The Pantashara and the Bozdag/Duzdag sections in Transcaucasia (Azerbaijan) were also proposed as neostratotypes (Danukalova, 1996).
The Akchagylian deposits are widespread in the Caspian area and their distribution shows the maximum extent of the Akchagylian transgression (Fig. 3b). The maximum thickness of the Akchagylian (up to 750 m) is reached in the deep Caspian basins whereas it is generally much thinner (0.5-10 m) in the periphery of the basin. In several sec-tions of Azerbaijan (Lokbatan, Jeirankechmez) the Akchagylian overlies the deltaic deposits of the Productive Series of the South Caspian Basin (Van Baak et al., 2013; Van Baak, 2015). In eastern Georgia, (e.g., Kvabebi section), the Akchagylian is deposited above the so-called pre-Akchagylian unconformity (Buleishvili, 1960), indicating syntectonic deformation in the western Kura Basin (seeAdamia et al., 2017). It is angularly unconformable and paraconformable at some points. Ak-chagylian deposits are overlain by conglomerate and sandy units there. Palynological data of the northern regions show dominating taiga for-ests (Kuznetsova, 1966) whereas the southwestern regions contain pollen that indicate open steppe landscapes (Kovalenko, 1971;Naidina and Richards, 2016).
The Akchagylian mollusc faunas are best characterised by the high number of endemic mactrid and cardiid bivalve species (Danukalova, 1996). The Akchagylian is often subdivided into three substages, based on its mollusc assemblages (Golubyatnikov, 1904, 1908;Kolesnikov, 1940;Ali-Zade, 1954;Yakchemovich et al., 1970;Paramonova, 1994). The lower substage is marked by low variety of genera and species containing Aktschagylia subcaspia, A. karabugasica, A. inostrantzevi, Cerastoderma dombra dombra. The middle substage is characterized by high species numbers within the genera Aktschagylia, Andrussovi-cardium, Miricardium and Avicardium. The upper substage is character-ized by low numbers of mollusc species including Aktschagylia sub-caspia, A. ossoskovi, Cerastoderma dombra dombra and species of Dreissena and Theodoxus. The gastropods Pirenella caspia s.l., “Clessi-niola” intermedia, “C.” utvensis, and “C.” vexatilis occur in all substages. The usage of this threefold scheme may lead to an arbitrary allocation of units, especially as it is only based on high or low mollusc species richness. For example, other investigations demonstrated that middle Akchagylian molluscs appeared at different stratigraphical levels, which led to an alternative subdivision of the Akchagylian in two substages (Popov, 1969; Trubikhin, 1977; Danukalova, 1996; Nevesskaya et al., 2003).
The lower part of the Akchagylian is marked by a sudden occurrence of euryhaline marine foraminifera like Cassidulina crassa, C. prima, possibly C. reniforme, and C. obtusa, Ammonia beccarii, Cibicides loba-tulus, Spirillina sp., Discorbis multicameratus, Miliolina aksaica
(Agalarova, 1976;Richards et al., 2018) as well as small sized biserial Bolivinidae. The latter are nowadays considered to belong to the planktonic genus Streptochilus (Smart and Thomas, 2006, 2007). During this marine influx, the freshwater dominated Pliocene ostracod fauna became replaced by an anomalohaline (“brackish”) water assemblage consisting of Limnocythere alveolata, L. luculenta, L. tschaplyinae, Typh-locypris gracilis, Loxoconcha eichwaldi, Candona candida, and C. combibo (Van Baak et al., 2013;Fig. 4). The upper part of the Akchagylian is characterized by the presence of Eucythere naphtatscholana, Amnicythere andrussovi, A. nata, A. multituberculata, A. cymbula, Leptocythere gubkini, Euxinocythere praebosqueti, Loxoconcha babazananica, and Camptocypria acronasuta.
2.1.1.2. Correlation. The age of the Akchagylian is mainly based on paleomagnetic investigations and correlation to the Geomagnetic Polarity Time Scale (GPTS) in combination with radio-isotopic dating. The interpreted ages of the boundaries are subject of debate. One of the first paleomagnetic results was obtained from western Turkmenistan and indicated that the onset of the Akchagylian in the South Caspian Basin started slightly below a normal-reversed boundary (Khramov, 1960, 1963). We consider this now the Gauss-Matuyama boundary, which corresponds to an age of 2.58 Ma. Note here that the first geomagnetic polarity time scale ever published, appeared later (Cox et al., 1963). Magnetostratigraphic investigations of numerous sections in Turkmenistan and Azerbaijan confirmed this N-R polarity pattern straddling the base of the Akchagylian (Trubikhin, 1977). The Pyrnuar section in western Turkmenistan, however, showed two small reversed
zones in the lower Akchagylian, which were correlated with chrons C2An.2r (Mammoth) and C2An.1r (Kaena) by Trubikhin (1977). Consequently, the base of the Akchagylian was matched to the base of the Gauss chron (C2An) at an age of 3.6 Ma, a correlation that has been accepted in numerous publications (Trubikhin, 1977;Semenenko and Pevzner, 1979; Sidnev, 1985; Molostovsky, 1997). Recently, Gurarii (2015) re-investigated the polarity pattern of the Pyrnuar section and concluded that the two reversed zones are too small to be interpreted as Mammoth and Kaena and that the base of the Akchagylian correlates better with the upper part of the Gauss chron (between 3.0 and 2.6 Ma).
In the northern Pre-Caspian region, the base of the Akchagylian Stage has been placed at the lower Nunivak chron (C3n.2n) at an age of ~4.5 Ma, but the main Akchagylian transgression event there was also dated at the Gauss-Matuyama boundary (Yakhemovich et al., 1981, 2000). Hence, it was suggested to correlate the lower/upper substage boundary with the Gauss/Matuyama reversal (Nevesskaya et al., 2005). The Akchagylian deposits of the Azov region and the Northern Greater Caucasus only show a single normal to reverse polarity change, inter-preted as the Gauss-Matuyama boundary, although the base Akchagy-lian is tentatively placed at the Gilbert-Gauss reversal at 3.6 Ma (Naidina and Richards, 2016).
The ash layers from the lowermost Akchagylian of Azerbaijan have been dated by Chumakov (Chumakov et al., 1988) using a fission track method at 3.34 ± 0.5 Ma, although they later reported that these ages were not very accurate and had to be confirmed by new data (Chumakov et al., 1992). Magnetostratigraphic data from the Lokbatan Fig. 3. Paleogeographic maps for the Plio-Pleistocene Pontocaspian region. A) Middle Pliocene; B) Late Pliocene; C) Early Pleistocene; D) Middle Pleistocene. Based onVinogradov, 1961, 1969andAbdurakhmanov et al. (2002).
section were not straightforward but led to the conclusion that the lower Akchagylian boundary should be around ~3.2 Ma (Van Baak et al., 2013). This was in good agreement with results from the Kvabebi section in eastern Georgia where the uppermost two normal subchrons of Gauss were attributed to the Akchagylian (Agustí et al., 2009). In the Kushkuna section of Azerbaijan, however, the lowermost Akchagylian only contains one normal polarity zone, correlative to the top Gauss (Trubikhin, 1977). Recently, the Akchagylian ash layers of the Lok-batan and Jeirankechmez sections of Azerbaijan were radio-isotopically dated with the40Ar/39Ar method, which resulted in a revised age of 2.7 Ma for the base of the Akchagylian in Azerbaijan (Van Baak, 2015). This is in excellent agreement with the magnetic polarity pattern of the Jeirankechmez section that shows the two reversed subchrons of the Mammoth and Kaena in the upper part of the Productive Series (Khramov, 1963; Van Baak, 2015). The earlier magnetostratigraphic correlation of Lokbatan was thereby rejected (Van Baak, 2015).
In conclusion, most evidence and interpretations converge towards two different ages for the base of the Akchagylian (Fig. 5). The “classic option” mainly relies on the magnetostratigraphic correlation of the Pyrnuar section and dates the base of the Akchagylian to the base of the Gauss at an age of 3.6 Ma. This is the officially accepted age in Russian stratigraphy (Provisions, 2003;Nevesskaya et al., 2005). The “young
option” mainly depends on sections in Azerbaijan, where the integra-tion of40Ar/39Ar dating and magnetostratigraphy indicates an age of 2.7 Ma for the base of the Akchagylian, in the uppermost normal part of the Gauss chron (Khramov, 1960;Van Baak, 2015).
2.1.2. Apsheronian
2.1.2.1. Description. Caspian Sea conditions in the Early Pleistocene Apsheronian were similar to today. The Caspian Basin was occupied by an anomalohaline lake. A major salinity decrease during the late Akchagylian corresponds with the almost complete extinction of characteristic Akchagylian bivalve faunas and subsequently the typical Apsheronian fauna evolved. This endemic fauna became increasingly dominated by extant endemic Caspian groups. During the Apsheronian, the Caspian Basin was mostly an isolated basin that may have had rare, short-lived connections only to the Black Sea via the Manych Strait (Fig. 3c).
The Apsheronian stage was established based on outcrops on the Apsheronian peninsula (Fig. 2) near Baku (Azerbaijan), and the Bailov Cap district was proposed as lectostratotype locality (Andrusov, 1923; Nevesskaya, 1975b). There, the Apsheronian deposits conformably overlie the Akchagylian deposits and are overlain, with an angular unconformity, by Bakunian strata (Nevesskaya, 1975b; Stratigraphy, Fig. 4. Distribution of the main ostracod species, foraminifera and charophyta in the Caspian Basin during the Plio-Pleistocene, based on literature data. 1) Cyprideis torosa; 2) Ilyocypris gibba; 3) I. bradyi; 4) Cyprinotus salinus; 5) Eucypris sp.; 6) Pseudocandona compressa (juvenile); 7) Limnocythere aralensis; 8) Zonocypris membranae; 9) Darwinula stevensoni;10) Limnocythere alveolata; 11) L. luculenta; 12) L. tschaplyinae; 13) Typhlocypris gracilis; 14) Loxoconcha eichwaldi (different stages of evo-lution); 15) Candona candida; 16) C. combibo; 17) Eucythere naphtatscholana; 18a) Amnicythere andrussovi; 18b) A. palimpsesta; 18c) A. normalis; 18d) A. saljanica; 19) Amnicythere nata; 20) Leptocythere gubkini; 21) A.multituberculata; 22) Euxinocythere praebosqueti; 23) A. cymbula; 24) Loxoconcha babazananica; 25) L. petasa; 26) Camptocypria acronasuta; 27) Caspiocypris filona; 28) Tyrrhenocythere bailovi; 29) Xestoleberis chanakovi; 30) T. amnicola donetziensis; 31) T. azerbaidjanica; 32) T. papillosa; 33) Camptocypria acronasuta; 34) Bacunella dorsoarcuata; 35) Loxoconcha gibboides; 36) L. lepida; 37) Euxinocythere bosqueti; 38) Cytherissa bogatschovi; 39) Euxinocythere bacuana; 40) A. quinquetuberculata; 41) Loxoconcha endocarpus
1982).
The size of the Caspian Sea slightly decreased compared to the Akchagylian, so Apsheronian deposits are, almost everywhere, found conformably overlying Akchagylian strata (Stratigraphy, 1982). Paly-nological data mainly reveal treeless landscapes, indicating an in-tensification of the continental climate and an increasing aridity when compared to the Akchagylian (Naidina and Richards, 2016).
The Apsheronian Stage is subdivided into three substages, based on changes in the composition of the mollusc fauna (Andrusov, 1923; Kolesnikov, 1940;Zhidovinov et al., 2000; Sidnev, 1985). The lower substage is characterized by a species-poor assemblage of low saline to fresh water bivalves (Dreissena, Corbicula, Apsheronia) and gastropods (Lymnaea, Streptocerella, Turricaspia, Theodoxus). The middle substage is marked by the first occurrence of the mollusс genera Parapsheronia and Didacna. The upper zone is characterized by a general depletion of the saline fauna and the disappearance of the ribbed apsheronids of the middle interval. This upper (Tyurkyanian) interval is poorly
recognizable in many parts of the Caspian Basin and in some regions only a twofold division has been used (Stratigraphy, 1982).
Euryhaline foraminifera of the genus Ammonia spp. have been ob-served at the base of the Apsheronian, suggesting a brief increase in salinity. Most of the ostracod species from the Akchagylian continued to evolve within the Apsheronian stage but became more frequent. The Apsheronian comprises the ostracod genera Leptocythere and Caspiolla; the middle Apsheronian is marked by a widespread radiation within the genus Leptocythere (Fig. 4). Tyrrhenocythere azerbaidjanica, T. papillosa, T. bailovi, Cyprideis torosa and Cytherissa bogatschovi are also commonly observed (Fig. 4).
The Tyurkyanian stage was defined byKhain (1950)for the con-tinental deposits (up to 100 m) between the Apsheronian and Bakunian successions (Stratigraphy, 1984). These deposits contain fresh water molluscs (Viviparus diluvianus, Valvata piscinalis, V. antiqua, Bithynia sp., Lithoglyphus naticoides, Pisidium amnicum, P. cf. supinum, P. cf. sub-truncatum, Sphaerium rivicola, Unio sp., Dreissena sp.) and correspond to
Fig. 5. Plio-Quaternary time scales for the Pontocaspian domain. GPTS with Systems and Stages are afterHilgen et al. (2012), oxygen isotope curve with numbered
Marine Isotope Stages (MIS) is afterLisiecki and Raymo (2005). M=Mammoth (C2An.2r), K=Kaena (C2An.1r), R= Reunion (C2r.1n), O=Olduvai (C2n), C.M.= Cobb Mountain (C1r.2n), J=Jaramillo (C1r.1n). Ages of the Cobb Mountain Subchron are afterChannell (2017). On the right side are the time scales for the Caspian Basin, Black Sea Basin and the terrestrial domain (mammal (MN/MQ) zonation and regional (MNR/MQR) biochronological units).
a strong (~150 m;Fig. 6) regressional phase (Danukalova et al., 2016; Svitoch, 2013a; Svitoch, 2013b; Yanina, 2012a; Yanina, 2012b; Zastrozhnov et al., 2018). In the fluvio-lacustrine Tyurkyanian deposits, ostracods typical of shallow fresh water environments are dominant (e.g., Ilyocypris bradyi, I. gibba, Pseudocandona compressa, Candona ne-glecta, Cypris subglobosa, Darwinula stevensoni, Eucypris sp.). The Tyur-kyanian deposits are well-expressed in the eastern margin of the South Caspian Basin and are locally present in the North Caspian Basin where they are known from drill cores and have a maximum thickness of 38 m (Danukalova et al., 2016). Here, we consider the Tyurkyanian as the upper part of the Apsheronian.
2.1.2.2. Correlation. An intensive paleomagnetic campaign to study the Apsheronian deposits in the Pontocaspian region took place in between 1960 and 1980 (Khramov, 1960, 1963; Trubikhin, 1977; Kochegura and Zubakov, 1978; Semenenko and Pevzner, 1979; Yakhemovich et al., 1981). The magnetostratigraphic correlations of sections in Azerbaijan and Turkmenistan were straightforward and have placed the Akchagylian/Apsheronian boundary at the base of a normal polarity interval, that was assumed to correspond to the Olduvai subchron (C2n), corresponding to an age of 1.95 Ma (Kochegura and Zubakov, 1978; Semenenko and Pevzner, 1979; Sidnev, 1985). Recently, multidisciplinary investigations based on integration of paleomagnetic, Ar/Ar dating, ostracod faunal analysis and lithological changes have been conducted again in the Kura Basin of Azerbaijan and
found the base of the Apsheronian close to, but slightly below the base of the Olduvai (Van Baak et al., 2013). Paleomagnetic data from the Duzdag section in Azerbaijan confirm that the base of the Apsheronian corresponds to a normal polarity interval (Pevzner, 1986). The magnetic polarity pattern at Duzdag, however, indicates that this normal polarity interval correlates to the Reunion subchron rather than to the Olduvai, giving a slightly older date of ~2.1 Ma for the basal Apsheronian.
Further revisions of the boundary have been proposed based on the new definition of the Neogene-Quaternary boundary. In 1998, the Interdepartmental Stratigraphic Committee of Russia (ISC) established the Neogene-Quaternary boundary on Russian territory at the top of the Olduvai subchron corresponding to an age of 1.8 Ma (Provisions, 1998). With an aim of correlating the Russian national stratigraphic chart to the global Geological Time Scale, the Akchagylian/Apsheronian boundary was made equal to the Neogene-Quaternary boundary, and therefore arrived at an age of 1.8 Ma. Since this time, many time scales in the Pontocaspian region place the base of the Apsheronian at the top of the Olduvai subchron (Nevesskaya et al., 2005).
The Tyurkyanian deposits are suggested to correspond to the low-ermost part of the Middle Pleistocene (Semenenko and Pevzner, 1979; Trubikhin, 1977). According to fission track data, however, the age of the Tyurkyanian is 1050 to 950 ka (Ganzey, 1984).
In conclusion, two different ages exist for the top of the Akchagylian/base of the Apsheronian (Fig. 5). The official geological
Fig. 6. Schematic reconstruction of the Black Sea and Caspian Sea water-level curves in comparison to global oxygen isotopes records ofLisiecki and Raymo (2005)
during the Pleistocene to Holocene. Associated interbasinal water exchanges marked by arrows pointing to the right for Mediterranean waters flooding into the Black Sea, arrows pointing to the left for Caspian Sea waters flooding into the Black Sea and double arrows for bidirectional water exchange between Black Sea and Caspian Sea. N.B. Two options exist in literature regarding the position of the Singilian: *Svitoch (2013band references therein) and **Zastrozhnov et al. (2018 and references therein).
time scale of Russia dates the Akchagylian/Apsheronian boundary at 1.8 Ma, resulting in a “long Akchagylian” of 1.8 Myr. The alternative correlation, based on the Duzdag section of Azerbaijan, dates this boundary at > 2.1 Ma, suggesting a “short Akchagylian” of 0.6 Myr if the age of 2.7 for the base Akchagylian in Azerbaijan is used, or an “intermediate Akchagylian” of 1.5 Myr if the age of 3.6 Ma for the base of the Akchagylian is used (Fig. 5).
2.1.3. Bakunian
2.1.3.1. Description. Conditions in the Middle Pleistocene Bakunian stage resembled the modern Caspian Sea in size, fauna and salinity regimes (Fig. 3d). During highstands punctuated overflows towards the Black Sea existed. Widespread carbonate rocks formed during warmer phases of the Bakunian in the South Caspian Basin; these are the main source for building material there.
The Bakunian Stage was defined bySjögren (1891)and comprises the sedimentary strata that conformably overlie the Apsheronian de-posits in the eastern part of the Apsheron peninsula of Azerbaijan (Fig. 2). Sjögren (1891) did not propose any stratotype and Golubyatnikov (1904)suggested the section Gora Bakinskogo Yarusa on the Apsheron Peninsula, which was later thoroughly studied by other researchers (Nalivkin, 1914; Fedorov, 1957; Mamedov and Aleskerov, 1988; Svitoch et al., 1992; Svitoch and Yanina, 1997; Yanina, 2005). The Neftyanaya Balka section was suggested as an ad-ditional reference section (Yanina, 2012b, 2013). Neftyanaya Balka is situated in the Kura Basin and has the advantage that it contains well defined stratigraphic boundaries for the Bakunian stage. The mollusc fauna of the Bakunian stratotype is characterised by a number of bi-valve species including Didacna parvula, D. catillus, D. rudis, D. cardi-toides, D. eulachia, D. mingetschaurica, D. pravoslavlevi, D. lindleyi. The taxa D. parvula and D. catillus are index-species, while D. rudis and D. carditoides are considered characteristic species (Bogachev, 1932a, 1932b; Fedorov, 1957, 1978a; Vekilov, 1969; Svitoch et al., 1992; Yanina, 2005, 2013;Nevesskaya, 2007;Svitoch and Yanina, 2007).
Sediments of the Bakunian are widespread on most Caspian coasts. In tectonic depressions, Bakunian deposits are deeply buried, and are only exposed in local uplifted structures. Sediments represent a range of facies and their thickness varies between a few meters on high terraces, to hundreds of meters (maximum thickness ~500 m) in the depressions. Spores and pollen indicate a relatively cold and humid climate in the early Bakunian, with forest formation (birch, alder, oak, maple, elm) on the western coast (Abramova, 1974;Filippova, 1997) and forest-steppe assemblages (pine, birch, alder, elm) in the lower Volga region (Moskvitin, 1962). The late Bakunian was marked by moderately warm and humid climate, as expressed by forest-steppes (oak, caracas, hop-hornbeam) in the Kura Basin (Svitoch et al., 1998), widespread ever-greens in Dagestan (Abramova, 1974), and steppes in the valleys of the Ural river (Yakhemovich et al., 1986).
The base of the Bakunian is determined by a significant transgres-sion that reached its maximum extent in the first half of the Middle Pleistocene (=lower Neopleistocene), but was much smaller than the Akchagylian transgression (Fig. 3c). The presence of Bakunian Didacna parvula, D. rudis, D. carditoides, D. catillus in the Black Sea (Chaudian Stage) and in the Manych Depression (Fedorov, 1978a;Popov, 1983; Svitoch et al., 1998, 2010; Yanina, 2005, 2006) represents Caspian overflows into the Black Sea Basin through the Manych Strait.
The Bakunian Stage is commonly subdivided into two substages based on different mollusc fauna, in particular the bivalve genus Didacna (Fedorov, 1957, 1978a; Moskvitin, 1962; Vekilov, 1969; Popov, 1983; Mamedov and Aleskerov, 1988; Yanina, 2005, 2012b, 2013;Nevesskaya, 2007). Three morphological groups of the Didacna genus exist: the crassoidal, catilloidal and trigonoides groups (Svitoch, 1967;Yanina, 2005, 2013;Nevesskaya, 2007). The faunas of the lower Bakunian are dominated by the first two groups and furthermore in-cludes taxa like Didacna parvula, D. catillus and D. fedorovi. In addition, Dreissena rostriformis is widely distributed. The most characteristic
species are Didacna parvula and D. catillus. The upper Bakunian pre-dominantly contains didacnas of the transitional crassoidal-catilloidal group (D. rudis, D. carditoides) and crassoidal group (D. eulachia, D. mingetchaurica, D. pravoslavlevi, D. bacuana) and representatives of the catilloidal and trigonoidal groups are rare. The most characteristic species are Didacna rudis, D. carditoides and D. eulachia (Yanina, 2005, 2013;Nevesskaya, 2007).
The transgressive base of the Bakunian is marked by a level rich in euryhaline foraminifera, represented mainly by Ammonia spp., Nonion sp. and Cibicides lobatulus. The Bakunian ostracod community is domi-nated by endemic species adapted to unusual salinity settings such as Eucythere naphtatscholana, Loxoconcha eichwaldi, L. petasa, L. babaza-nanica, Tyrrhenocythere azerbaidjanica, T. papillosa, T. amnicola do-netziensis, Cytherissa bogatschovi and Bacunella dorsoarcuata (Fig. 4). Furthermore, many leptocytherid and candonid species occur. During the Bakunian, a morphological transformation of the carapace features took place, especially in loxoconchids. Several early stage morphotypes evolved into new species that characterize the present-day Caspian Sea ostracod fauna, including Loxoconcha endocarpus, L. lepida and L. gib-boides.
The Urundzhikian stage has been defined byFedorov (1946)as an independent stratigraphic unit corresponding to the final stage of the Bakunian transgressive cycle (Fedorov, 1993, 1999). The stratotype of the Urundzhikian is the section Yuzhny Urundzhik in Western Turk-menistan (Fedorov, 1946). As an additional reference section Neftya-naya Balka in Azerbaijan was suggested, which contains well defined stratigraphic boundaries (Svitoch and Yanina, 2007). Urundzhikian deposits are known from the southern Caspian Basin only (Kura Basin, Apsheron peninsula and southwest Turkmenistan). They are char-acterized by numerous Didacna species: D. eulachia, D. mingetschaurica, D. pravoslavlevi, D. colossea, D. shirvanica, D. bergi, D. karelini, D. por-sugelica, D. čelekenica, D. rudis, and D. carditoides. Trigonoidal and ca-tilloidal didacnas are rare. Characteristic species are D. eulachia, D. pravoslavlevi and D. kovalevskii. The size of the endorheic Urundzhikian lake phase slightly exceeded the area of the modern Caspian Sea. Here, we consider the Urundzhikian as the upper part of the Bakunian. 2.1.3.2. Correlation. The beginning of the Bakunian transgression is generally considered to correlate with the lower part of the Middle Pleistocene as defined on the International Quaternary Chart (Fedorov, 1978a; Rychagov, 1997; Yanina, 2012a, 2013; Svitoch, 2013a; Zastrozhnov et al., 2018). The age of Bakunian deposits is ~600 ka according to fission track analysis, and 378-480 ka according to thermoluminescent dating (Lavrishchev et al., 2011a, 2011b). Based on palynological data, the lower Bakunian is correlated to isotope stages MIS 18-16 (750-625 Ka) and the upper Bakunian to MIS 15-13 (625-475 ka). The large size and thickness of Didacna shells and the high carbonate content of Urundzhikian sediments suggest warm climatic conditions. The Urundzhikian transgression is consequently correlated to the interglacial stage MIS 11 (Svitoch and Yanina, 2007; Yanina, 2012a).
Magnetostratigraphic data indicate that the Bakunian pre-dominantly correlates with the normal Brunhes Chron, which provides a maximum age of 780 ka (Asadullayev and Pevzner, 1973). A recent study of the Xocashen section in western Azerbaijan places the Ap-sheronian-Bakunian boundary between the Jaramillo and Bruhnes chrons, in a reversed subchron correlative to chron C1r.1r with an age of 0.88-0.85 Ma (Van Baak et al., 2013), but these authors did not distinguish the Tyurkyanian.
In conclusion, the age of the lower Bakunian boundary is con-strained to the age interval between 0.88 and 0.75 Ma (Fig. 5). The official geological time scale of Russia places the Apsheronian/Baku-nian boundary at the Brunhes/Matuyama reversal at an age of 0.78 Ma. 2.1.4. Khazarian
Hyrcanian phases the Caspian Sea experienced large sea level changes and episodic overflow into the Black Sea occurred (Fig. 6). In between these high-stands, deep regressions took place and essentially the modern Caspian system evolved.
The Khazarian Stage has been defined by Andrusov (in Pravoslavlev, 1913) and comprises the sedimentary strata that con-formably overlie Bakunian deposits in the North Caspian Basin and the Manych Strait.Fedorov (1953)suggested a twofold subdivision into a lower and upper Khazarian, which is officially accepted in Russian lit-erature. Lower Khazarian deposits in boreholes on the Apsheron pe-ninsula were also called “Gyurgyanian” (Dashevskaya, 1936, 1940), a name that can be found in some stratigraphical schemes (e.g., Nevesskaya, 2007). Many researchers have considered another unit in the northern Pre-Caspian area, the Singilian, either at the base of the lower Khazarian (Pravoslavlev, 1913, 1932; Fedorov, 1957, 1978a; Moskvitin, 1962; Goretskiy, 1966; Rychagov, 1997), or as an in-dependent unit between Bakunian and lower Khazarian intervals (Sedaikin, 1988; Svitoch and Yanina, 1997; Yanina, 2012b;Svitoch, 2013b). In these scenarios the Singilian is considered time-equivalent with the Urundzhikian (Fig. 6).Popov (1983), however, considered the Singilian as the regressive phase of the lower Khazarian. More recently, the regressional Singilian phase was placed between the early and late Khazarian transgressions (Zastrozhnov et al., 2018).
The early Khazarian transgression corresponds to a sea level high stand of +15 m (Fedorov, 1957;Vasiliev, 1961;Rychagov, 1997) or +20-25 m (Svitoch and Yanina, 2007). The lower Khazarian deposits are widespread along the Caspian coast, penetrating far inland along paleodepressions. The sediment thickness ranges from a few meters to thick sequences in the depressions. The maximum thickness is recorded in the Kura Basin, where it reaches 600 m (Vekilov, 1969). Spore and pollen data indicate a three-stage change in vegetation: from goosefoot-sagebrush steppe at the beginning of early Khazarian time, to taiga type forests in its middle and periglacial steppe at the end (Zastrozhnov et al., 2018).
The lower Khazarian is characterized by the broad development of the trigonoidal group of didacnas (Didacna subpyramidata, D. paleo-trigonoides, D. gurganica, D. mishovdagica, D. trigonula, D. trigonoides chazarica), representatives of the crassoidal group (Didacna pra-voslavlevi, D. nalivkini, D. delenda, D. apscheronica, D. ovatocrassa, D. subcrassa, D. pontocaspia tanaitica) and rare species of the catilloidal group (Didacna dilatata, D. subcatillus, D. vulgaris, D. lindleyi, D. adac-noides). Characteristic species for the lower Khazarian are Didacna subpyramidata and D. paleotrigonoides. Numerous other anomalohaline molluscs are present as well (Monodacna caspia, Hypanis plicatus, Adacna vitrea, etc.). Typical ostracods are Caspiolla gracilis, Candoniella subellipsoida, Leptocythere arevina, L. martha, L. quinquetuberculata, L. gibboida, Bacunella dorsoarcuata, Loxoconcha endocarpa, Cyprideis littor-alis and Cytherissa cascusa (Ushko and Shneider, 1960;Sedaikin, 1988; Svitoch and Yanina, 1997). Among foraminifera, Ammonia caspica and Mayerella brotzkajae occur (Yanko, 1989).
The early Khazarian spans several interglacials and glacials. Khazarian deposits of the Manych Depression include two overflow phases (Fedorov, 1978a; Svitoch and Yanina, 1997; Yanina, 2005, 2012a). The early Khazarian transgressions mostly developed under relatively cold climate conditions but some of the transgressions de-veloped during interglacial intervals. Traces of permafrost are found in deposits of the Northern Pre-Caspian region, which represented a periglacial interval during glacial intervals (Grichuk, 1954;Moskvitin, 1962;Zhidovinov et al., 1984). Deciduous woods developed in Dage-stan (Abramova, 1974). Glaciers existed in the Caucasus mountains and cold and humid conditions existed in Azerbaijan during these glacials (Milanovsky, 1966;Dumitrashko et al., 1977;Aleskerov, 1990). Lower Khazarian spores and pollen from boreholes in the Southern Caspian are dominated by grassy plants and trees (pine, fir, birch, alder, willow) (Vronsky, 1976).
The late Khazarian transgression (Fig. 7a) corresponds to a sea level
high stand of -10 m (Kaplin et al., 1977a;Popov, 1983; Svitoch and Yanina, 1997), but lacked any connection with the Black Sea Basin (Fig. 6). Common gigantism of shells, high carbonate content in the sediment, and the presence of oolites indicates warm climate conditions (Yanina, 2014). Salinities ranged between 10 to 12‰ in the northern part and up to 14-15‰ in the southern part of the Caspian Basin, i.e. higher than today (Yanina, 2014). Pollen assemblages testify to mod-erately warm interglacial climates (Abramova, 1974).
The upper Khazarian is marked by widespread species of the cras-soidal Didacna group (Didacna surachanica, D. subcrassa, D. hyrcana, D. nalivkini, D. delenda, D. ovalis, D. karabugasica, D. subovalis, D. ovato-crassa, D. schuraosenica). Trigonoidal and cattiloidal forms are rare. The index species of the upper Khazarian is Didacna surachanica (Fedorov, 1957;Yanina, 2005;Nevesskaya, 2007). In more oligohaline and fresh-water deposits Corbicula fluminalis is common.
Foraminifera are represented by Ammonia novoeuxinica, A. caspica, Elphidium capsicum, E. shochinae, Frorilus trochospiralis, Majerella brotz-kajae, Cornuspira minusculla and Milioniella risilla (Yanko, 1989). The Khazarian ostracod fauna is a continuation of the Bakunian fauna (Fig. 4). The most common species are Candona schweyeri, Fabae-formiscandona sp., Eucythere naphtatscholana, Cyprideis torosa, Lox-oconcha eichwaldi, L. petasa, L. lepida, L. babazananica, Tyrrhenocythere azerbaidjanica, T. papillosa, T. amnicola donetziensis, Euxinocythere ba-cuana, Amnicythere andrussovi (and its associated morphotypes), A. striatocostata, Bacunella dorsoarcuata and Cytherissa bogatschovi (Sedaikin, 1988).
After the main late Khazarian transgression, but before the Khvalynian transgression, another transgression has been described from boreholes in the North Caspian Basin; the Hyrcanian (or Girkanian) transgression (Fig. 6;Popov, 1955, 1967;Goretskiy, 1957; Yanina, 2013; Sorokin et al., 2018). Hyrcanian deposits contain “Khvalynian-like” fauna of Didacna subcatillus, D. cristata, D. pallasi, D. subcrassa, but also the mainly freshwater Corbicula fluminalis. The widespread occurrence of C. fluminalis is indicative of the warm water character of the basin. The Hyrcanian mollusc fauna in the Manych Depression shows that Caspian waters were draining to the Black Sea (coevally with the last phase of the Karangatian transgression in that basin) through the Manych Strait (Popov, 1983;Yanina, 2014).
The Khazarian sediments (including the Hyrcanian in their upper part) are generally separated from the lower Khvalynian deposits by the Atelian regression (Fig. 6), when the Caspian Sea level was significantly lowered (Fig. 7b). Based on seismic-acoustic profiling, the maximum lowstand during the Atelian is estimated at -120 to -140 m (Lokhin and Maev, 1990;Maev, 1994). Vast areas of the Caspian shelf were exposed and river incisions were deep (Fedorov, 1978a;Rychagov, 1997). Ate-lian deposits contain mammal remains of the Upper Paleolithic faunal complex, including mammoth, horse, reindeer, etc., indicative of tundra-steppe and cold arid continental climate. Towards the end of the Atelian epoch, the climate became even warmer. In the vegetation, the share of arboreal pollen (birch, pine and spruce trees) increased while elm, oak and linden re-appeared. The importance of xerophytes de-creased, while grasses and herbaceous vegetation expanded. Steppe and forest-steppe environments became dominant (Grichuk, 1954; Chiguryaeva and Khvalina, 1961; Moskvitin, 1962; Bolikhovskaya et al., 2017).
2.1.4.2. Correlation. The lower Khazarian is generally considered to be equal to the upper part of the Middle Pleistocene, while the upper Khazarian is correlated with the lower part of the Upper Pleistocene. According to Th-U, TL and electron spin resonance data, lower Khazarian deposits are dated at ages of > 300, > 250, 148-177, and 142-108 ka. They have a normal polarity (Brunhes) and one or two sub-zones of reverse polarity (Shkatova, 2010). The Khazarian comprises three transgressive stages which correspond to MIS 10, 8 and 6 (Fig. 6; Yanina, 2014).
127-87 and 122-(130)-84 ka (Shkatova, 2010), 144-90 ka (Geochronology…, 1974), 130-91 ka (Leontiev et al., 1975), and 122-106 ka (Shakhovets, 1987;Shkatova, 2010). According to Uranium-Ionium dates, the age of the late Khazarian transgression was estimated at between 114 and 75 ka (Leontiev et al., 1975;Rychagov, 1997), and between 115 and 81 ka (Arslanov et al., 1978). Results of electronic paramagnetic (spin) re-sonance (ESR) revealed ages between 140 and 85 ka (Bolikhovskaya and Molodkov, 1999, 2008;Molodkov and Bolikhovskaya, 2009) and 108-85 ka (Shkatova, 2010). These results led to the conclusion that the late Khazarian transgression phase took place between 127 and 122 ka, the initial regression phase at 117–114 ka BP, and the full regression phase at 114–85 ka, coeval with MIS 5e, 5e-d, and 5d-a, respectively (Shkatova et al., 2010). Late Khazarian and Hyrcanian deposits of the Srednyaya Akhtuba section (lower Volga area) were OSL dated at 112 ± 5 ka, 102 ± 5, 87 ± 4 ka indicating they also correspond to MIS 5 (Yanina et al., 2017a, 2017b). Paleomagnetic measurements of upper Khazarian sediments confirmed the occurrence of a reversal excursion in five sections. Its age (from Th–U, TL and ESR measurements) is es-timated between 117 ± 7.5 and 89 ± 11 ka (Shkatova, 2010), and thus is most likely to correspond to the reversed polarity Blake event (~120 ka). It is generally accepted that the late Khazarian transgression cor-responds to the Eemian Interglacial.
Most age constraints indicate that the upper Khazarian deposits
have an age between 125-85 ka, corresponding to MIS 5 (Fig. 8). The Hyrcanian transgression corresponds to the final part of the interglacial interval MIS 5a (85 ka). The Atelian regression then most likely peaked at an age of 85-75 ka. The final phase of regression is dated by OSL at 48 ± 3 (Yanina et al., 2017b) and by 14C at 45–41 ka (Bezrodnykh et al., 2017) suggesting it corresponds to the first half of the interstadial warming of MIS 3.
2.1.5. Khvalynian
2.1.5.1. Description. The Khvalynian stage developed in the Late Pleistocene glacial period. Very high transgressions with an overflow event towards the Black Sea interchanged with very deep regressions. Salinities were somewhat depressed compared to today and a unique landscape (Baer knolls) developed in the North Caspian plains.
The Khvalynian Stage has been defined by Andrusov (in Pravoslavlev, 1913) and comprises the sedimentary strata that trans-gressively overlie the Atelian and Khazarian deposits in the North Caspian Basin and Manych Strait. The Khvalynian transgression is by far the most extensive sea-level rise in the Late Pleistocene history of the Caspian Sea (e.g.,Yanina, 2014) (Fig. 7c). Khvalynian deposits are generally subdivided into lower and upper Khvalynian highstand in-tervals that are separated by the Enotaevka regression (Fig. 6).
The well preserved paleocoastlines of the early Khvalynian Sea Fig. 7. Paleogeographic maps for the late Pleistocene Pontocaspian region. Arrows indicate the water flow direction in the gateway regions. All maps are based on Yanina (2014).
allow precise determination of facies distributions. Its sedimentary thickness is usually only a few meters, but can reach over 100 m in the Kura and Western Turkmenistan basins. The lower Khvalynian deposits comprise widely different sediments, including the characteristic liman-type “chocolate clays” of the lower Volga area that probably formed by accumulation of fine brown sediments that derived from the periglacial landscapes of the hinterland (Moskvitin, 1962; Goretskiy, 1966; Makshaev and Svitoch, 2016;Tudryn et al., 2016). Palynological data confirm a cold climate (Abramova, 1974;Yakhemovich et al., 1986). The early Khvalynian water level reached +50m, and Caspian Sea water spilled over to the Black Sea via the Manych Strait (e.g.,Popov, 1983;Varuschenko et al., 1987;Yanina, 2014). During this period the North Caspian Basin was much larger than today (Fig. 7c).
The lower Khvalynian and upper Khvalynian are separated by the Enotaevka regression (Brotsky and Karandeeva, 1953), which reached a maximum lowstand at -105 m (Maev, 1994). Terrestrial Enotaevka deposits and numerous unconformities are described from the coastal regions (Leontiev and Fedorov, 1953;Vasiliev, 1961;Rychagov, 1974; Fedorov, 1978a;Svitoch and Yanina, 1997). The regressive Enotaevka unit in the North Caspian Basin consists of deltaic deposits with marked cross-bedding (Roslyakov et al., 2007). It is prominent in the deeper parts and pinches out towards the shore where it is characterized by an unconformity (e.g. Svitoch and Yanina, 1997). According to pollen data, arid cool climate conditions existed (Sorokin et al., 1983). During the late Khvalynian transgressive stage, sea level reached about 0 m (which is 27 m above todays Caspian levels: (Fedorov, 1957; Varuschenko et al., 1987;Rychagov, 1997). No overflow to the Black Sea existed. Upper Khvalynian deposits are found on all Caspian coasts
in the interval of -20 to 0 m, but their thickness does not exceed a few meters.
In the lower Khvalynian Didacna ebersini, D. parallella, D. protracta are abundant, while D. cristata, D. subcatillus, D. praetrigonoides, D. de-lenda and D. zhukovi are rare. Characteristic species are D. parallella and D. protracta. On the eastern coast, these species are replaced by D. cristata and D. zhukovi. The thin shells of these molluscs indicate rela-tively low water temperatures compared to the present-day Caspian Sea. The structure of mollusc fauna is indicative of relatively low sali-nities, although regional variations existed. Salinities in the North Caspian Basin (3-4‰) were slightly higher than today, whereas sali-nities in the southern basin (~11 ‰) were slightly lower (Yanina, 2014). The upper Khvalynian deposits comprise clays, silts and sands with Monodacna caspia, Dreissena polymorpha, D. grimmi, and gastro-pods. Multiple Didacna species include D. praetrigonoides, D. parallella, D. protracta, and more rarely D. subcatillus. Didacna praetrigonoides, a rare species in the early Khvalynian, became dominant. The salinity of the main water body of the late Khvalynian basin was very similar to the early Khvalynian (Yanina, 2014). The relative abundance and thick shells of the molluscs indicate warmer conditions in late Khvalynian times then during early Khvalynian. Palynological data confirm a general warming of the Caspian region (Grichuk, 1954; Abramova, 1974; Vronskiy, 1974; Vronsky, 1976; Sorokin et al., 1983; Yakhemovich et al., 1986).
Ostracods of the Khvalynian are mainly represented by Loxoconcha gibboides, L. endocarpa, Caspiolla gracilis, Leptocythere propinqua, L. martha, Paracyprideis enucleate, Cyprideis torosa (Sedaikin, 1988). For-aminifera are dominantly brackish water species (Yanko, 1989). 2.1.5.2. Correlation. The ages of the Khvalynian transgressions are rather ambiguous and remain subject of discussion (Kaplin et al., 1972, 1977a, 1977b; Kvasov, 1975; Leontiev et al., 1975; Arslanov et al., 1978, 2016; Svitoch and Yanina, 1997; Rychagov, 1997; Bezrodnykh et al., 2004; Badyukova, 2007; Yanina, 2014). Based mostly on (now considered outdated) TL dates, the age of the lower Khvalynian deposits was estimated at 70-40 ka, and the age of the upper Khvalynian deposits at 20-10 ka (Leontiev et al., 1975;Rychagov, 1997). According to14C and230Th/234U data the age of the Khvalynian is 19-8 ka (Kvasov, 1975; Svitoch and Yanina, 1997; Leonov et al., 2002;Chepalyga et al., 2008;Svitoch et al., 2008;Tudryn et al., 2013; Arslanov et al., 2016). Based on the radiocarbon dates from drilling the North Caspian Basin, lower Khvalynian ages are 30-21 ka and upper Khvalynian ages 19-12 ka (Bezrodnykh et al., 2004, 2015). The OSL dating of the chocolate clay in the lower Volga area resulted in ages of 15 ± 1 and 13 ± 0.5 ka (Yanina et al., 2017b). Paleomagnetic studies of the lower and upper Khvalynian sediments revealed the presence of two magnetic excursions, identified at several sections (Seroglazka, Lenino, and Yenotaevka), which are likely to correspond to the Laschamps (41 ka) or Mono Lake (32 ka) events (Laj and Channell, 2007). The discrepancies between these results from different methods, which may even lead to inversions in the chronology, show that the ages for these transgressions are not yet well established (Tudryn et al., 2013).
According to Sorokin et al. (2018), the lower Khvalynian trans-gression correlates with the global interstadial warming of the younger half of MIS 3 at an age of 30-21 ka, and is caused by an increase in the surface runoff from the catchment area. The Khvalynian sea level rise was interrupted at the LGM (time of maximum cooling and aridization, MIS 2) and resumed when the ice sheet was decaying. The warm phases of Bølling and Allerød promoted ice sheet melting along with thawing of permafrost, the latter having been widespread in the Volga drainage basin (Sorokin et al., 2018). The Khvalynian came to its end at the first sharp warming that resulted in the rise of the Caspian level and is generally taken as marking the Pleistocene/Holocene boundary.
In conclusion, it seems that most age constraints for the base of the Khvalynian converge on an age of 35 ka (young option), while the top is 3 4 5 LR04 δ18O (‰) 1 2 6 10 12 14 16 18 20 5 7 9 11 15 8 A psh. Chaudian U zunlar ian K ar anga tian Neoeuxinian Chernom-n K haz a rian K h v alynian Novocaspian BLA CK SEA BASIN C ASPIAN SEA BASIN Br u hn e s GPT S 2012 C alabr ian
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ua
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Tar an tian Bak unian Holocene S y st em Subser ie s/ S tages 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 A ge , M a?
Fig. 8. Quaternary time scales for the Pontocaspian domain. GPTS with Systems and Stages is afterHilgen et al. (2012), oxygen isotope curve with numbered Marine Isotope Stages (MIS) is afterLisiecki and Raymo (2005). On the right side are the time scales for the Caspian Basin and the Black Sea Basin.
estimated at ~10 ka (Fig. 8). The age of the maximum Enotaevka re-gression separating the lower and upper Khvalynian is around ~15 ka. 2.1.6. Novocaspian
2.1.6.1. Description. The Holocene Novocaspian stage represents the modern Caspian Sea settings and faunas.
The Novocaspian (Holocene) Stage, defined by Bogachev (1903), comprises the sedimentary strata that transgressively overlie the Khvalynian deposits in the North Caspian Basin. Novocaspian deposits are developed on all Caspian coasts below -19 (-20) m. In the North Caspian Basin, they are represented by shallow water sands with nu-merous fresh-and anomahaline molluscs species. The Novocaspian is separated from the upper Khvalynian by the regressive Mangyshlakian facies (defined by Zhukov, 1945), that represents the deltaic pro-gradation of the Volga and Ural rivers during a major sea level fall up to – 80 m or even – 113 m (Figs. 6, 7d) (Varuschenko et al., 1987; Bezrodnykh et al., 2004, 2015). The Mangyshlakian deposits contain peats and sands with plant detritus and species poor assemblages of fresh water and oligohaline molluscs (Dreissena polymorpha, Lymnaea, Unio) but lack Didacna species (Sorokin, 2011).
The Novocaspian mollusc fauna is marked by various Didacna spe-cies of the crassoidal and trigonoidal groups: Didacna crassa, D. baeri, D. trigonoides, D. pyramidata, D. longipes, D. barbotdemarnii. In addition, numerous other anomalohaline molluscs are present like Monodacna caspia, Hypanis plicatus, Adacna vitrea, etc. Characteristic for the Novocaspian are the species D. baeri and D. trigonoides, plus the entry of Cerastoderma glaucum (Fedorov, 1953, 1957). The uppermost beds contain Mytilaster minimus and Abra segmentum, which were anthro-pogenically introduced into the Caspian from the Azov/Black Sea during the 20th century. Spores and pollen spectra contain up to 25% of tree pollen, mainly pine and birch, indicative of the relatively humid climate in the Holocene (Sorokin, 2011).
2.1.6.2. Correlation. Judging from the absolute age determinations, the Mangyshlakian regression peaked between 10 and 8 ka, and the Novocaspian deposits are all younger than 7 ka (Fig. 8). The AMS14C data from boreholes in the Volga delta suggest a lowstand around 8000 BP. Volga delta data indicate a continuously rising sea level between 5000 and 3000 BP until a highstand was reached at -25 m around 2600 BP (Overeem et al., 2003; Kroonenberg et al., 2008). During the historically well-known mediaeval Derbent regression (Rychagov, 1997) Caspian Sea levels dropped to -34 m, and possibly even -45 m as recorded in the deeper parts of the offshore Kura delta in Azerbaijan. A second highstand around 300 BP is documented in an outermost barrier in Dagestan (Kroonenberg et al., 2007). The two highstands appear to coincide with two well-known periods of increased precipitation in Eurasia, the 2600 BP event (Van Geel and Renssen, 1998) and the Little Ice Age, whereas the Derbent regression seems to be coeval with the Warm Mediaeval Period (Kroonenberg et al., 2007). 2.2. The Black Sea region
The Black Sea is today a marginal sea of the Mediterranean (Fig. 2) and has a surface area of 436,400 km2(excluding the Sea of Azov), a maximum depth of 2,212 m, and a volume of 547,000 km3. Its E-W extent is about 1175 km (27°27'-41°42') and it stretches ~800 km N-S (46°33'- 40°56'). At present, it is the world's largest meromictic water body; deep waters do not mix with the upper water layers that receive oxygen from the atmosphere. As a result, over 90% of the deep Black Sea volume is anoxic. Circulation patterns are primarily controlled by basin topography and fluvial inputs, which result in a strongly stratified vertical structure. The Black Sea has a positive freshwater balance: it receives more fresh water from the rivers and rainfall than it loses from evaporation. The Black Sea consequently experiences an estuarine type of water transfer with the Mediterranean Sea via a shallow threshold (35-40 m) at the Bosphorus Strait, with bottom inflow of dense
Mediterranean water below a surface outflow of fresh Black Sea water into the Marmara Sea. The salty Mediterranean inflow mixes with the basin’s fresher waters, which results in an average salinity of 18 – 22‰ for the Black Sea surface waters, i.e. much lower than the Mediterra-nean (37 – 38‰).
During Quaternary to Recent times, periods of isolation and episodic connection with the Mediterranean Sea (through the Marmara and Aegean seas) largely controlled the paleoenvironmental conditions in the Black Sea (Zubakov, 1988;Badertscher et al., 2011;Van Baak et al., 2016b). The ancient Bosphorus Strait may have been slightly deeper than today, as the sill depth in the Paleozoic bed rock is estimated at ~85 m in the Dardanelles Strait (Algan et al., 2001). However, the Bosphorus gateway itself evolved only in the Middle Pleistocene (McHugh et al., 2008). When global/Mediterranean Sea levels were above the Bosphorus sill, marine water connections existed all the way to the Black Sea. During such connection phases the Black Sea level tracked that of global sea levels. When the Mediterranean levels were below the sill, the Black Sea turned into an isolated, saline lake basin. Major rivers like the Danube, Dniester, Dnieper, and Don (via the Sea of Azov), supply fresh water to the Black Sea; together they drain a large part of continental Europe (Fig. 2). During intervals with a positive water balance the Black Sea level remained at the sill height and one-directional flow towards the Mediterranean Sea occurred. In times of negative water budgets, lake levels dropped until the total inflow (precipitation and river influx) equalled the evaporation in the Black Sea Basin (e.g.,de la Vara et al., 2016). Salinities during these lake phases were typically in the oligohaline-mesohaline ranges, very si-milar to today’s Caspian Sea.
In the northeast, the Black Sea is connected to the Sea of Azov through the Kerch Strait (Fig. 2). The Sea of Azov (45°12′-47°17′N, 33°38′-39°18′ E) has a surface area of 39,100 km2and is 13 m deep in its central part. Two large rivers, the Don and Kuban, flow into the Sea of Azov. Annually 49.2 km3waters flows to the Black Sea, while 33.8 km3returns, resulting in an average salinity of ~11 ‰. Recently, an-thropogenic reduction of river drainage strengthened the inflow of the Black Sea waters and increased the average salinity up to 13.8 ‰. In the geological history, the Sea of Azov frequently desiccated during glacio-eustatic lowstands and the Don and Kuban rivers directly drained into the Black Sea, south of the modern Kerch Strait (Yanina, 2012a).
During the Pontian (6.1-5.6 Ma), the Black Sea was connected to the Mediterranean in the south, the Dacian Basin of Romania in the west and to the Caspian Basin in the east (Popov et al., 2004, 2006; Krijgsman et al., 2010;Van Baak et al., 2015b, 2016a, 2017). During early Pliocene times (regional Kimmerian age) the Black Sea Basin became isolated and transformed into a long-lived lake (Fig. 3a). During the late Pliocene-early Quaternary (Kuyalnikian), modern Pontocaspian faunal elements appeared in the Black Sea Basin, where a succession of saline lake stages developed, increasingly punctuated (from the Middle Pleistocene onwards) by short marine connectivity phases similar as today. Here we review the stratigraphic development of the Black Sea domain during the Plio-Quaternary.
2.2.1. Kuyalnikian
2.2.1.1. Description. At the onset of the Pleistocene, the Pontocaspian domain consisted of two autonomous provinces; the mesohaline-polyhaline Akchagylian basin of the Caspian region and the oligohaline-mesohaline Kuyalnikian basin of the Black Sea region (Fig. 3b). The late Pliocene-early Pleistocene Kuyalnikian Beds (Sintsov, 1875) are composed of shallow-water sands, sandstones, and clays with marly interbeds and represent a stratigraphic analogue of the Akchagylian sediments. They represent an anomalohaline, mostly isolated long-lived lake in the Black Sea Basin.
The Kuyalnikian (or Kujalnician) is named after the “Kuyalnik Estuary” north of Odessa in the Ukraine (Fig. 2) and inherited its brackish-water regime from the preceding Pontian and Kimmerian water bodies of the Black Sea, after which the basin gradually
